Stray crystal formation in Al-20Â wt.% Cu studied by synchrotron X-ray video microscopy

Recent improvements in detectors combined with the eminent brightness and collimation offered at 3rd generation synchrotron sources have opened the field for in situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching true video microscopy. Here, observations on dendrite fragmentation from columnar fronts in Al-20 wt.% Cu and subsequent transport phenomena will be presented. From directional solidification experiments, it has been found that the frequency for crystal fragmentation, resulting from remelting of dendrite arm roots, depends strongly upon the buoyant forces. At 20 wt.% Cu concentration, primary aluminium dendrites are lighter than the melt and solidification experiments parallel- and anti parallel to gravity show significant differences in stray crystal formation tendency. Under particular conditions, it has been found that crystal fragmentation could lead to a meso-scale liquid segregation.